The way it works is that it checks the campaign status. If the player can proceed to the next mission and all primary goals are complete, it plays the success music. If the player can proceed and not all primary goals are complete, it plays the quasi-success/failure music. If the player cannot proceed, it plays the failure music.
In the mission simulator, the player can never proceed to the next mission (since there is no next mission). So the failure music always plays.
It would be a very simple thing to fix - just test to see whether the player is in a campaign or not. If the player isn't in a campaign, then play the success music if all primary goals are complete and the quasi-success/failure music if they aren't.